eco prelim exams
first difficulty depends on school and how harsh they a) set multiple choice and b) how harsh they mark essays
prepping for eco prelim exams highly recommend constantly revising your content by building mindmaps, and doing hard multiple choice questions. start gathering a stat bank now, and if you haven't already, generally pay attention to the news, specifically anything politics, economics and business, that will come in handy.
practice essay writing ideally from now, and get your teacher (or tutor, apparently economics tutoring is starting to be a thing, i find it stupid) to mark it for you, and constantly get feedback. when writing essays remember that although you may be taught a formula (TEEL ,PEEL, APEEL, OREO, etc), an essay is simply just an argument you make, and when writing an essay, you want to at the end of the day be setting out in clear logical steps how everything works, using examples to back you up. This is imperative when writing essays, fluency and clarity of your argument ALWAYS COMES BEFORE whatever formula you are using, if it makes sense to bend or break the formula, do it.
I don't think prelim (or even hsc exams) get too difficult, for social sciences generally its about knowing the content really damn well and knowing how to apply it, its not like there are like crazy tricks you just got to know like you do for 3u and especially 4u maths.